4 Answers
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"The Aionomica, the Rammahgon, a dozen other mystic-sounding made-up names—the foundation of the ancient faith." ―Luke Skywalker

It's referring to the fact that the 8 books Rey takes from the tree library are extremely knowledgeable books in terms of Force knowledge, including information on "Chain World Theorem" (The World Between Worlds), Wayfinders, and the unsolved Thorpe Theorem: a hyperspace plotting conundrum. You can read more about the books, such as the Rammahgon, here, as it was included/explained in The Rise of Skywalker Visual Dictionary.

It's also referring to the idea of Yoda's restored sense of faith in the Force. During the course of the original trilogy, Yoda had exiled himself on Dagobah after he felt he greatly failed the Jedi. When Obi-Wan brings young Luke to him, he is initially unimpressed and doesn't really believe that Luke has what it takes to become a Jedi or save the Galaxy. By the time we get to the sequel trilogy and after Yoda has died and returned as a Force Ghost, the role echoes, as the past nearly repeats and Luke has exiled himself on Ahch-to, because he felt he failed the Jedi again. But Yoda's viewpoint since had changed in terms of not loosing faith in both Luke (who he once again prods to do the right thing) and Rey. In other words, he has faith that The Force will guide Rey and that she will be successful in her endeavors. The line about outgrowing what we have learned, is about being willing to 'let go' and trust in the new generation's capacity to learn from the Force beyond whatever anyone before them had tried to teach them.

Wisdom they held, but that library contained nothing that the girl Rey does not already possess.

This is a double entendre by Yoda.

The figurative meaning is that Rey is already wise enough, perhaps due to her time on the island, that she does not need the books anymore. This meaning appears to be what Yoda intends because of the "but", and it makes Luke feel better about the loss of the books.

The literal meaning is that Rey has physically taken the books with her. In the next shot we see that this is indeed the case. This is what Yoda actually means.

Yoda: Wisdom they held, but that library contained nothing that the girl Rey does not already possess.

This line by Yoda is a hint that Rey has already taken the Ancient Texts from the temple before it burnt down. We see them aboard the Falcon at the end of the film when Finn is looking through some drawers.

And of course we see Rey has them in her hideout and reads them in The Rise of Skywalker.

Hmm, while that makes sense, I had seen the statement quite a bit broader as Rey having internalized what it really means to be a Jedi without needing a bunch of old psalms for that.
– Napoleon Wilson♦Mar 30 at 13:19

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@NapoleonWilson If that were true she wouldn't have been relying on them so heavily in TRoS.
– TheLethalCarrotMar 30 at 13:21

In addition to the double entendre mentioned in other answers (that Yoda knew that Rey had literally taken the books with her), and Yoda's belief that Rey would successfully learn to use the Force with or without the books, another interpretation is that Yoda believes that being a Jedi is more about character and intent than any specific scholastic tradition. This would represent a departure from Yoda's earlier characterization of being fairly inflexible and dogmatic towards Jedi tradition (initially refusing to train Anakin due to his age, demanding that Luke continue his training rather than save his friends, etc.), which is why it's shocking to the viewer that he initially appears to be burning the books himself.

The theme of masters burning their sacred texts once enlightenment has been achieved appears in multiple Zen koans, which inspired some aspects of the Force that appear in the other films. For example:

Once there was a well known philosopher and scholar who devoted himself to the study of Zen for many years. On the day that he finally attained enlightenment, he took all of his books out into the yard, and burned them all.

The Zen master Mu-nan had only one successor. His name was Shoju. After Shoju had completed his study of Zen, Mu-nan called him into his room. "I am getting old," he said, "and as far as I know, Shoju, you are the only one who will carry on this teaching. Here is a book. It has been passed down from master to master for seven generations. I also have added many points according to my understanding. The book is very valuable, and I am giving it to you to represent your successorship."

"If the book is such an important thing, you had better keep it," Shoju replied. "I received your Zen without writing and am satisfied with it as it is."

"I know that," said Mu-nan. "Even so, this work has been carried from master to master for seven generations, so you may keep it as a symbol of having received the teaching. Here."

The two happened to be talking before a brazier. The instant Shoju felt the book in his hands he thrust it into the flaming coals. He had no lust for possessions.

Mu-nan, who never had been angry before, yelled: "What are you doing!"

In these stories, the lesson is that while tradition and collected teachings may help achieve wisdom, they are not themselves the end goal of wisdom. This seems to be what Yoda is saying, that the practice of doing the right thing is more important than mastery of theory:

Page-turners they were not. Yes, yes, yes. Wisdom they held, but that library contained nothing that the girl Rey does not already possess. Skywalker, still looking to the horizon. Never here, now, hmmm? (pokes Luke with his walking stick) The need in front of your nose. Hmmm?

(Note that this lesson is somewhat undercut by Rey having the actual books, and later using them to fix her lightsaber and track down Exegol in The Rise of Skywalker.)

In addition to this being a nice answer, I also share your sentiment in the last sentence. I found the idea of reorienting on what it really means to be a Jedi and doing away with the institutionalization that lead to their downfall in favour of a more intuitive spiritual approach to the Force a really interesting aspect of the film and very much in line with the general motifs of doing away with the old in favour of the new. So I was a bit disappointed when it turns out Rey actually saved the tomes.
– Napoleon Wilson♦Mar 31 at 17:38

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But I do think it isn't that much of a contrast to see Yoda doing that. This contrast primarily comes from having the prequels in mind as his last and most prominent appearance, where he indeed might have been a lot more...strict in his ways. But the Yoda from The Last Jedi was actually more a return to the...playful Yoda from the old films, who knew to prefer wits over knowledge, likely after realizing how the Jedi failed in the old republic.
– Napoleon Wilson♦Mar 31 at 17:45

@NapoleonWilson Good point; he's less tradition-bound in the OT than in the PT. In TLJ he seems closest to his characterization in ROTJ, where he cracks irreverent jokes about how he's old and can't do anything about dying, even in the face of Luke's concern. In TESB, he does spend a while talking about how being a Jedi requires "the most serious mind" and admonishes Luke's "recklessness", but also tests Luke's character in a very practical way by pretending to be crazy, rather than some kind of solemn ritual.
– Milo PMar 31 at 17:52

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Just to add to this nicely laid out answer on Yoda's characterization, the new upcoming High Republic era is also going to [eventually] inform why the Jedi of the prequel era are the way they were, as the Jedi from this era 200 years before, are meant to be a bit different: polygon.com/star-wars/2020/3/11/21172493/…
– Darth LockeMar 31 at 18:03